Primary design

[YoungBart]

Hi all,
Don't think I'm duplicating here but if I am feel free to delete,
my boss asked me to work out the size of a SWA feeding an extension to a commercial property, basically the contractor wants to get the cable in, before the building design has been finalised, so we're basically playing it safe with the cable size, basically allowing X amount of Watts per meter for lighting and small power, allowing X amount of KVA for mechanical gear, X amount of KVA for kitchen gear etc. then to work out the size of the cable from there, Basically I need someone to tell me if I'm on the right lines, as my boss has had to go out tomorrow so isn't able to check and I don't want to base the rest of my day on the wrong calcs.
Heres what I did,
find the length of the cable run,
find the floor area of the new extension,
Multiply the watts per meter by the floor area. Then convert that to Amps using the 3phase conversion method,
Convert the KVA to amps again using the 3ph method
total up the load, then size the cable accordingly (taking into account lengths, and other factors)
The bit I wasn't really sure about was converting the Watts and KVA to 3ph or Single phase.

I apologise for the long, possibly confusing thread, but if anyone can tell me if I'm going in roughly the right direction that'd be great.

 

[telectrix]

you can't really size your cable without knowing roughly what is going to be installed. if you don't have a clue what the load is going to be, look at your main supply fuses. if they are 100A per phase, then you could calculate the cable size as having a max load of 100A/phase, then calculate the VD from that and the length. it might end up being overkill, but you won't be under.

 

[YoungBart]

We've got a rough idea as it's based on a similar install, and we've added plenty to that to ensure safety, I was more wondering if my calculation method was about right

 

[Richard Burns]

Possibly a bit late now.
As this is a very rough approximation erring on the side of caution then you can be fairly straightforward about it.
I assume power factor here is irrelevant as this is covered by your safety margin.
For each cable you need to know the current running in each core of the cable as this is what will determine your cable size.
If you have the power from the lights and sockets as you have said and add this to the total kVA from the other areas and assume (probably inaccurately) that it is a totally balanced load then I=P/V gives you total current as if single phase, divide by 3 to get current per core.
Take account of the installation method of the cable.
From the length calculate volt drop as this is your primary limitation and you want it to be low so that the final circuits can account for most of the volt drop, hopefully you will find the cable needs to be larger than the CCC would indicate so gives more leeway if you have no voltage sensitive equipment.

 

[Farmelectrics]

Always anoys me when customers clients don't tell you the maximum demand it's that that you design to they end up paying a lot more money for a bigger cable that they don't need

 

[YoungBart]

Thanks for the replies guys, it's all sorted now, cable oversized but I suppose it's better that way than undersized.

 

[telectrix]

so. what size cable have you selected?

 

[Richard Burns]

Thanks for the replies guys, it's all sorted now, cable oversized but I suppose it's better that way than undersized.

You have not oversized the cable, what you have done is designed the installation with built in future proofing!

 

[YoungBart]

It's a 95mm Cable over a length of around 130 meters, in an underground duct, Unfortunately I cant remember the final load I came up with, I think it was around 400A But I could be wrong